CA2146278C

CA2146278C - Back-up material and method for drilling printed circuit boards

Info

Publication number: CA2146278C
Application number: CA002146278A
Authority: CA
Inventors: Marshall A. Janoff
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-10-27
Filing date: 1995-04-04
Publication date: 2001-06-19
Anticipated expiration: 2015-04-04
Also published as: US5716168A; CA2146278A1

Abstract

A back-up board for use in drilling holes in printed circuit boards is disclosed.
The inventive back-up board comprises outer layers of paper impregnated with a high-density resin, and a core consisting of alternating layers of paper impregnated with low-density resin and dry paper.

Description

21~s27~
-- , Fxpreas Mail No.

B.~CK-L~P 1L~TERL~L A\D ~1ETHOD FOR
DRI1:.LL1G PR>L1'TED CIRCUIT BOARDS
TECHNICAL F'fELD
This im~ention relates to back-up materials, and more particularly, to laminated back-up materials used when drilling holes in printed circuit boards..
io BACKGROUND ART
r The drilling of holes in single sidad, double sided, andlor multilayer circuit roard lanunates is a particularly critical machining op:ration. Good quality holes are essential to producing high quality circuit boards. ~ good quality hole is one that is produced ha~~n~
15 nW imal defects. Such defects typically include resin smear, r~aiI heading, rough hole walls, and inaccuracy in the size or position of the hole on the board. The drillir~
operation itstlf can result in problems v~ith the holes, such as surface burrs and epoxy smears. ~ complete solution to these problems requires the utiliution of a board which is attachtd to the back of the circuit board during the hole drifting operation (a "back-up board'').
so Most back-up boards are made «7th a vbod core which contains chips and flakes of cellulose and resin which are lantinated with phenolic, melamine, epoxy or aluminum corer sheets. slthouQ,,.lt better than many other products, these materials can leave dust in the drilled holes, aluminum galled to the drill tip, or can cause e~:cessive drill wear.
Solid phertolic back-up boards tend to be very dense and abrasive. They can also Z3 eause hole-wall degradation from chip-pressure built up in the drilling process, and cart chip and ~~ ear the drill bits.

2146~'~8 Softer woody, like medium density fiberboard (~SDF), can be aery dusty, and are not hard enough to prevent burrinS of the bottom panel of the circuit board being drilled.
The hardness of these woods generally range from b5-75 shore-d. although this material will minimize drill wear, it is not applicable for the higher aspect ratio drilling which is required by many users.
LS. Patent ~'o. ~4,8~3,273 discloses a back-up board comprising a corrugated core material made of saturated Kraft paper. The problem with the bZcl:-up board of '373.
however, is that upon lamination, the corrugated core tends to collapse, thus dastro~ing the purpose and integrity ofthe core. Additionally, the back-up board of'373 is a to thermoszt resin impregnated core ~~hich fuses upon lamination. Such fusion is undesirable because it increases the core density, thereby increasing the level of contamination Icft in the hole when the dnll bit is removed.
The back-up board disclosed in U.S. Patent~o. 3,700,341 employ; a core of pressed good, such as masonite, covered W th a metahic surface, such as aluminum. The t3 ' masonite, however, is too dense of a ms~terial, and results in exctssi~e abrasion and wear to the drill bits. Use of mu.~otlite also tends to lease debris in the hole.
~Othet methods have been de~~ised in an attempt to eliminate these problems. For example, U.S.
Patent \'o.
4,269,549 utilizes a metallic corrugated core. The cost of such a core material, however, is prohibitive.

21462'78 It is an object of the present im~ention so pro~7de an improved bacl.-up board for use in drilling printed circuit boards.
It is another object of the prcscnt im~cntion to provide an impro~~ed method of r manufacturing a back-up board for use in drilling printed circuit boards.
It is another object of the present invention to provide a back-up board which is cost efr~ctiae and which allows the drilling of claan, burr-free holes in printed circuit boards.
It is yet another object of the present intention to prolong the useful lift of drill to bits used for drilling holes in printed circuit boards.
Still other objects and advantages of the present im~ention W 11 become readily apparent to those skillad in this art from the followvtg detailed description, vrherein tha preferred embodiment of the invention is shown and described simply by way of illustration of the best mode comernplated for carrying out the invention.

DISCLOSURE OF T>iI~ ~NTTON
According to the present im~ention, the foregoing objecti~~as and ad~-antages are attained by s back-up board comprising outer layers constructed of paper impregnated v~ith a high-density resin which results in a hud surface, and a core consisting of layers of o dry paper and paper impregnated with a low-density resin. The im~entive back-up board's hard outer shell resuhs in the drilling of bun-free circuit board holes, while the low-density _ resin/dry paper core prevents wood dust and debris from accumulating in the holes and reduces drill wear.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 composite side view of drill penetrating entry, circuit multiple boards, and back-up board.
Flg. 2 side sectional view of the components which form the sheet of laminated drill back-up material.
Flg. 3 exploded view of the individual back-up board sheets prior to lamination.
Flg. 4 a cross-sectional view of how the printed circuit board drill enters into the back-up material after drilling through a stack of circuit board laminates.
Fig. 5 an illustration of how printed circuit board drill design exacerbates drill chip compression as debris is transported up the drill flutes.
Flg. 6 top view of drill bits generally ranging in size from #52 - #68, normally referred to as "heart-of-the-range", naming the geometrical components thereof.
Flg. 7 side view of drill bits generally ranging in size from #52 - #68, normally referred to as "heart-of-the-range", naming the geometrical components thereof.
Flgs. 8A - 8B cross sections of drill showing flute volume at the tip and at the exit point of drill bits generally ranging in size f rom #52 - #68, normally referred to as "heart-of-the-range', naming the geometrical components thereof.

- Fig. 9 top view of drill bits generally ranging in size from #69 and smaller, normally referred to as "small-hole drills", naming the geometrical components thereof.
Fig. 10 side view of drill bits generally ranging in size from #69 and smaller, normally referred to as "small-hole drills', naming the geometrical components thereof.
FIgS. 11A - 11B cross sections of drill showing flute volume at the tip and at the exit point of drill bits generally ranging in size from #69 and smaller, normally referred to as "small-hole drills", naming the geometrical components thereof.
DETAILED DESCRIPTION OF THE INVENTION
Flg. 2 illustrates the back-up board of the present invention. The invention comprises a plurality of laminated planar sheets on either side of a dry paper core. Outer layers 1 and 7 are resin-impregnated to provide a relatively hard surface on which the circuit board can be supported during drilling. The outer layers each have a thickness of approximately .006", and are made of paper impregnated with a resin, such as melamine, to provide a hard surface. Central layers 2, 4 and 6 are made of paper impregnated with a softer resin, such as phenolic. The invention is not limited to using only these two resin systems. Core layers 3 and 5 are made of dry (unimpregnated) Kraft paper. The low-density central layers 2, 4 and 6 and dry core 3 and 5 each have a thickness of approximately .011".
The layers shown in Flg. 2 consist of: melamine-impregnated Kraft paper 1;
phenolic-impregnated Kraft paper 2; dry Kraft paper 3; phenolic-impregnated Kraft paper 4; dry Kraft paper 5; phenolic-impregnated Kraft paper 6; melamine-impregnated Kraft :b k2 _. paper 7. The backup board has a hardness, as measured by a durometer, of approximately 95 shore-d. The preferred thickness of the backup board of the present invention ranges from approximately .060" - .075". However, the invention is not limited to this range. If a greater thickness in the back-up board is required, additional alternating sheets of low-density resin-impregnated paper and dry Kraft paper may be employed. The specific number of layers utilized is a function of the required thickness of the finished laminate.
The inventive back-up board may be manufactured by at least two different processes: high pressure platen pressing or continuous pressing. In the high pressure platen method of manufacture, Kraft paper sheets are pre-impregnated in their respective resins to provide the appropriate hardness. Then, as shown in Fig. 3, sheets of impregnated and dry paper are stacked from top layer to bottom layer as follows: melamine impregnated Kraft paper 1A; phenolic impregnated Kraft paper 2A; dry Kraft paper 3A;
phenolic impregnated Kraft paper 4A; dry Kraft paper 5A; phenolic impregnated Kraft paper 6A; melamine impregnated Kraft paper 7A. The specific number of sheets utilized may vary depending upon the required thickness of the finished laminate.
After stacking the paper in this order, the stack of paper is then pressed under high pressure and temperature. It is preferred that the back-up board have a high glossy finish.
This result is achieved by pressing the stack with a #8 mirror plate, finished caul sheets, and/or platens. The pressure used in this operation is generally in excess of 350 pounds per square inch (psi), more properly from 800 -1000 psi. The temperatures will vary depending on the specific resin systems and the needed times for the resins to properly 21462'8 thermoses (cure). This process »ill result in the production of a back-up material which has a hard outer surface, and a lon~-densityldry paper inner core.
Furthermore, this method of manufacture «i11 result in a back-up board which may be peeled apart or separated due to the special and unique nature of the board's construction and lamination.
r 3 Ifthe manufacturing process is to be conducted on a continuous press, then rolls of hard resin pre-impregnated paper are collated with a less dense resin-impregnated paper and dry Kraft paper, as shown in Fig. 3, 1 ~ ti-,rough 7 ~. :again, the individual iaytrs are stacked in the following order (from top to bottom): melamine impregnated Kraft paper 1 ~; phenolic impregnated Kraft paper ?..~ dry Kraft paper 3 a; phenolic impregnated Kraft paper 4 ~; dry KraR paper S.i.,, phenolic impregnated Kraft paptr 6A;
melamine impregnated Kraft paper 7.~. As pith the plate;t press manufacturing process, the specific number of sheets utilized in the continuous press process is determined by the required thickness of the finished laminate. In the continuous press manufacturing process, both the resin-impregnated and dry paper sheets are draw through rollers which bring the sheets IS into a position to be heated. Heating is commonly done by a large heated drum set at a temperature sufficient to properly cure the resin.
In either method of manufacture, the following results are achieved: A) smooth hard surface sheets, both top 1 and bottom 7 »ith a mirror plate 5nish. The resultant combination of the pressure, thermosetting resins and the smooth plates will make the zo surface very hard: at least a 90 shorn-d durometer hardness reading is preferred. B) the core 2-6 will hare a lovyer density than the outer surface, and will not become a solid plastic upon lamination. The core may be peeled apart, since the dry sheets of Kraft paper will only partially absorb the resin from the adjoining resin-impregnated sheets.
The process of drilling printed circuit board laminate is conducted on sophisticated computer-driven machinery. This equipment must be very stable and accurate.
Newer technology uses spindles which rotate in excess of 100,000 revolutions per minute. Drill bits are generally made of solid "micro grain' carbide, with cobalt levels of 6%
through 10%o.
These drills are fragile and must be treated accordingly. Some drills are made by combining a stainless steel shank with a solid carbide flute.
Fig. 1 illustrates a drill penetrating through entry material 17, multiple circuit boards 18, and back-up board 19. The drilling operation itself is performed at very high speeds, and at relatively high rates of drill descent into the printed circuit board. Drill geometry plays a large role in the drilling process. Flgs. 4 anCl 5 show the basic structure of the drill bit 16. When drill bit 16 descends into entry material 17, circuit boards 18 and back-up board 19, there is a resultant increase in chip (or debris) pressure as the drilled material is transported up the flutes 13. As the drill descends and then ascends, the tip of the drill peels out large volumes 14 of material which is gradually and continually compressed by the web taper 10 as it travels towards the top of the drill 15.
This wedge effect not only compresses the debris, but it also creates heat in the process, and forces the debris against and into the hole wall. This effect can cause rough holes or epoxy smear - a condition where the epoxy melts due to excessive drilling temperatures. Should a blockage occur at the top of the drill, material could be extruded forward and blow out the bottom holes of the bottom board. Because it utilizes a low-density resin/dry ~146~78 paper core in the center, the back-up board of the prc5ent im'ention reduces the problems associated with chip pressure.
Additionally, the hard outer surface of the present im~ention will help to prevent bottom burring that may occur during drilling. The core of the im~ention is hard enough to r s support the surface, yet because of its lower density, it will nunimiza debris compression, resulting in cleaner holes. Used properly, the in~~entive back-up board hill reduce excessit'e drill wear. After the initial use, the im'entive back-up board may be turned oaer and tha bottom surface 7 utilized as the top of the back-up. Moreover, if the i~wentive back-up board is made to have a thickness of approximately .010" to .03.f', it may be to utilized as an entry board. Thus, the im'entiv=e back up board presents a very cost-e~'ecti~e alternative to ousting back-up boards.
The present invention imptoaes over know back-up boards, such as that shown in U.S. Patent \ro. 4,83,273, in that the inventive laminated core is rigid enough in construction to prevent core compression. This is important uthen drilling very deep small 1s holes, since high aspect ratio drilling results in enormous chip pressure whic>l may e.~ctrudo debris dowm the flutes instead of up. Figs. 9, 10, and 11 show~hotv'the design of small-hole drills tin e~ca~erate the compression as compared to heart-of-the-range drills in Figs. 6, 7, and 8. In high aspect ratio drilling, the compression caused by the drilling can cause the edge of the hole on the bottom panel to tear tnd burr.
Such an to occurrence is s major defect since the copper usually tears awsy from the epoxy v-all which bonds them together. Since glass bundles are e,cposed, plating becomes difficult, increasing the probability of defective connections in the circuit board. The invtmive 214~'~'~8 back-up board t~~il1 reduce burring by reducing the chip pressures caused by high aspect ratio drilling.
Vfiile preferred characteristics of the irn~ention have been disclosed, modifications to the disclosed embodiments of the invention, as well as other attributes thereof, may occur to thox skilled in tha art. Accordingly, the appended claims are intended to cover atl embodiments ofthe invention and modifications to the disclosed embodiments which do riot depart from the spirit and scope of tha im~ention.

Claims

1. A method of drilling holes through a circuit board with a drill bit and a back-up board having first and second outer planar sheets of resin pre-impregnated paper, a plurality of resin pre-impregnated central planar paper sheets disposed between the first and second outer planar sheets, and a plurality of planar dry paper sheets into which the drill bit descends and being respectively disposed between successive ones of said plurality of central planar paper sheets, and wherein the first and second outer sheets, the plurality of central sheets, and the dry paper sheets are bonded together under high pressure and temperature to create hard surface outer sheets having a higher density than the density of the combination of dry and resin impregnated central sheets, comprising the steps of:
positioning a back-up board beneath the circuit board with the first outer planar sheet disposed adjacent the circuit board; and drilling through the circuit board with said drill bit and into the back-up board by penetrating through said first outer planar sheet and through at least one of said central planar paper sheets and into at least one of said dry paper sheets.

2. The method of claim 1, wherein the resin in the pre-impregnated central planar sheets is softer than the resin in the pre-impregnated paper of the first and second outer planar sheets.

3. The method of claim 1, comprising the further step of penetrating with the drill bit through a second of said central planar paper sheets and through said second outer planar sheet.

4. The method of claim 1, comprising the further step of penetrating with the drill bit through a second of said central planar paper sheets and into a second of said dry paper sheets.

5. The method of claim 1, wherein said first outer planar sheet is approximately 6 mils thick, and each of said central planar paper sheet and said dry paper sheets are approximately 11 mils in thickness.